题目:
Given two words (beginWord and endWord), and a dictionary"s word list, find all shortest transformation sequence(s) from beginWord to endWord, such that:
Only one letter can be changed at a time
Each intermediate word must exist in the word list
For example,
Given:
beginWord = "hit"
endWord = "cog"
wordList = ["hot","dot","dog","lot","log"]
Return
[
["hit","hot","dot","dog","cog"], ["hit","hot","lot","log","cog"]
]
解答:
public class Solution { List> result; List
list; Map > map; public List > findLadders(String beginWord, String endWord, Set
wordList) { result = new ArrayList >(); if (wordList.size() == 0) return result; list = new LinkedList
(); map = new HashMap >(); int curt = 1, next = 0; boolean found = false; Set unvisited = new HashSet (wordList); Set visited = new HashSet (); Queue queue = new ArrayDeque (); queue.add(beginWord); unvisited.add(endWord); unvisited.remove(beginWord); //BFS while (!queue.isEmpty()) { String word = queue.poll(); curt--; for (int i = 0; i < word.length(); i++) { StringBuilder sb = new StringBuilder(word); for (char ch = "a"; ch <= "z"; ch++) { sb.setCharAt(i, ch); String newWord = sb.toString(); if (unvisited.contains(newWord)) { if (visited.add(newWord)) { next++; queue.add(newWord); } if (!map.containsKey(newWord)) { map.put(newWord, new LinkedList ()); } map.get(newWord).add(word); if (newWord.equals(endWord) && !found) found = true; } } } if (curt == 0) { if (found) break; curt = next; next = 0; unvisited.removeAll(visited); visited.clear(); } } backTrace(endWord, beginWord); return result; } public void backTrace(String word, String beginWord) { if (word.equals(beginWord)) { list.add(0, beginWord); result.add(new ArrayList (list)); list.remove(0); return; } list.add(0, word); if (map.get(word) != null) { for (String s : map.get(word)) { backTrace(s, beginWord); } } list.remove(0); } }
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